Applicants claim priority under 35 U.S.C xc2xa7119 of Austrian Application No. A1302/98 filed Jul. 29, 1998. Applicants also claim priority under 35 U.S.C. xc2xa7365 of PCT/AT99/00187 filed on Jul. 27, 1999. The international application under PCT article 21 (2) was not published in English.
The invention relates to a wrought aluminium alloy for a layer of a friction bearing which, apart from impurities inherent in the smelt, additionally contains Sn, Pb, Bi and Sb as soft-phase formers, as well as a multi-layered material made therefrom and a method of producing multi-layered materials incorporating at least one such aluminium alloy.
In order to avoid the disadvantages of lower resistance to fatigue inherent in aluminium-tin alloys containing silicon when used in plain bearings due to the notching effect of the silicon particles on the one hand and the cutting effect of the silicon particles in the region of the friction surface on the other, it is common practice to dispense with silicon when introducing alloying elements. One of the methods proposed (DE 4231 862 A1) as a means of improving the mechanical properties of silicon-free aluminium alloys with a high tin content, in the region of 35% by weight to 65% by weight, is to introduce lead and bismuth in a quantity of 0.5% by weight to 1.0% by weight in total on the one hand and at least one of the elements manganese, nickel, silver, magnesium, antimony and zinc on the other in a total quantity of at most 5% by weight. Because of the high tin content, as the smelt sets to an alloy, a cohesive tin network forms, which has a considerable adverse effect on the structural strength of the bearing material and its deformability, which is crucial with regard to the standard plating of these cast alloys with steel and the associated moulding steps. Furthermore, the higher the tin content is, the greater the effect the network structure of the tin in the aluminium matrix has on the mechanical properties of the material used for the plain bearing.
Aluminium alloys with a high tin content are also known from other publications. DE 4004703 A1, for example, discloses a coating material for bearing elements with an aluminium-based anti-friction coating. In addition to the usual permissible impurities, it contains added quantities of 1% by weight to 3% by weight of nickel, 0.5% by weight to 2.5% by weight of manganese and 0.02% to 1.5% by weight of copper. The tin content is 0.5% by weight to 20% by weight. This composition produces a tin phase in the form of dispersed tin particles in a matrix of an AINiMnCi crystal mixture alongside rolled-in hard particles. The added tin is intended to produce a friction-type bearing element which has as smooth as possible a running action, even at higher speeds, reduced friction and improved resistance to galling. However, this publication points out that if necessary, the added tin can be replaced by lead in a quantity of between 1% by weight and 10% by weight, which specifically rules out the use of an aluminium alloy of this type with an added high tin content in friction-type bearings for high-performance motors, due to the poor distribution of the tin phase.
A multiple-layer friction-type bearing having a coating of an aluminium-tin bearing alloy with a tin content of from 7% by weight to 20% by weight is known from DE 43 32 433 A1. Here, however, it is pointed out that the mechanical properties of the bearing alloy are adversely affected if the tin content is in excess of 20% by weight and a bearing alloy of this type cannot be used under tough conditions, for example in the case of a higher-performance motor. For this reason, silicon is incorporated in the alloying elements in a quantity of up to 4% by weight.
A bearing alloy with an aluminium-tin base having 7% by weight to 35% by weight of tin is known from DE 30 00 773 A1. The introduction of additional elements into the alloy is intended to improve the fatigue strength, whilst reducing the hardness at high temperatures and in particular avoiding any coarsening of the tin particles. The intention is also to increase resistance to wear of the bearing alloy in order to improve durability with regard to a shaft to be mounted. The disadvantage is that in order to obtain these properties, the aluminium alloys must contain a higher proportion of chromium, from 0.1% by weight to 1% by weight, in order to maintain the distribution of the tin.
A bearing alloy with an aluminium base is known form U.S. Pat. No. 4,471,032 A, to which between 1.5% by weight and 35% by weight of tin is added. In addition, this alloy contains between 1% by weight and 11% by weight of at least one element from the group consisting of manganese, iron, molybdenum, nickel, zirconium, cobalt, titanium, antimony, niobium and chromium, which again means that inter-metallic hard particles are formed, these being intended to improve the durability properties of a bearing made therefrom under more difficult conditions. The proportion of these hard phases in the matrix makes use with high-performance motors more difficult, however, because the desired lubricating effect of the tin can be reduced.
WO 97/122725 A describes an aluminium alloy, which simultaneously has a high tin content and exhibits a high strength. The latter is obtained amongst other things due to the formation of inter-metallic phases, which make the aluminium matrix more solid. The composition is selected so that the inter-metallic precipitations do not have a negative effect on the matrix strength. Furthermore, the specific wetting behaviour of these precipitations with the tin helps to improve the structural strengthxe2x80x94because the matrix structure is only negligibly disrupted by the tin network.
One aspect of this solution, the use of very little aluminium in soluble formers of inter-metallic phases, does however have a disadvantage in that no use is made of the potential hardening effects or the hardening effect which can be obtained is reproducible within only rather broad margins.
Accordingly, the underlying objective of the invention is to provide an aluminium alloy whose structural strength and mechanical properties can be improved, even in the presence of higher tin contents.
This objective is achieved by adding a quantity of at least to the first-described aluminium alloy. Wrought aluminium alloy for a layer in a friction bearing which, apart from impurities inherent in the smelt, additionally contains Sn, Pb, Bi, Sb as soft-phase formers, and a quantity of at least one element from the group of elements consisting of Sc, Y, Hf, Ta, La, lanthanides and actinides up to a maximum of 10% by weight, said quantity forming intermetallic A3M phases with the aluminium, said A3M phases being an average diameter of 0.005 xcexcm to 5 xcexcm. The advantage here is that it produces an Al-alloy which does not exhibit any marked hardening behaviour whilst exhibiting a high ductility due to the finely dispersed distribution of A3M-phases and that, in spite of the breakdown of solidified materials occurring during the manufacturing process due to heat treatments, high values of mechanical strength can be preserved. As a results a product can be produced which exhibits good thermal, static and dynamic stability. Another advantage is the fact that this Al-alloy or the material used to produce it, may have a high recrystallisation temperature, which means that heat treatments or deformation princesses can be performed at increased temperatures without giving rise to an undesirable reduction in hardness and so that friction-type bearings, for example, will also be capable of withstanding higher temperatures such as occur with new types of bearing elements with high-speed rotary shafts, for example. Yet another advantage is that because of the possibility of using multiple combinations of individual elements of the specified group material characteristic can be freely adjusted within specific limits, thereby enabling the inherent cost of producing the Al-alloy to be controlled accordingly. On the other hand, however, being able to introduce radioactive elements or isotopes such as U235 into the alloy simultaneously means that tracers can be incorporated in the alloy for test purposes so as to monitor the behaviour of the material on different test machines.
By adding Li, Zn, Si, Mg, Mn, Cu, Be, Ca, Zr, Mo, W, Ag, Ti, V, Cr, Fe, Co, Ni, Pd, Au, Pt, In, Ge, Sn, Pb, Sb, Bi and Te, or as a result of the large number of possible combinations, particularly when using the Al-alloy as an anti-friction layer for a bearing, the alloy can be readily adapted to specific requirements, in particular to the properties of the layers comprising the friction bearing. The effects which can be achieved by introducing the elements specified in these claims into the alloy can be taken from the detailed description below.
Advantageously, the Al-alloy for a layer in friction bearings, in addition to the requisite hardness, also contains up to 50% by weight of soft-phase formers dispersed in the Al matrix. Due t the elements of the Pb, Bi, Sb group, it is possible to act on the tension at the interface boundary of the tin so that when the Al-matrix hardens the tin can not precipitate on the grain boundaries of the matrix as a cohesive network. Interrupting the network structure of the soft phases, in particular the tin phase, brings about a change in the pattern structure, increasing the structural strength of the Al-alloy accordingly and improving deformability.
An Al-alloy with a high proportion of volume of soft phases and a desirable hardness, permitting its use in motors with high-speed rotating shafts and without premature fatigue, can be produced with an alloy whose hardness value after massive forming is no more than 35% below that value measured prior to a heat treatment for a period of 0.5 to 48 hours and at a temperature between 85xc2x0 C. and 445xc2x0 C., or 70% to 80% below that value measured prior to a heat treatment for a period of 1 to 24 hours and at a temperature between 100xc2x0 C. and 350xc2x0 C. The alloy has a preferred Vickers hardness between 25 HV2 and 80 HV2.
An advantageous multi-layered material for friction bearings, comprising at least two layers of differing compositions, in which the hardness of the layers is different and increases from a first peripheral layer to a second peripheral layer lying opposite it, has at least one of the peripheral layers made from the Al-alloy of this invention. The advantage of this is that due to the layout of the Al-alloy, multi-layered materials made from layers of a different composition, for friction bearings for example, can be produced such that their service life can be extended due to the improved mechanical properties or quality of the multi-layered material. As a result, the periods between any maintenance needed can be increased and, if the multi-layered material is used in a bearing application, the shafts supported thereby can be operated for a longer period without the worry of damage to the surface of these shafts due to inadvertent friction forces.
The objective of the invention is also achieved by a method of producing the above-indicated multi-layered material for friction bearings, wherein at least the first peripheral layer is joined to an intermediate supporting layer.
If the previously hardened first peripheral layer is rolled with at least one other Al base alloy, which may optionally contain other alloying elements or no soft-formers, the quality of the bond can be improved since the inherent properties are those of the aluminium base having several layers.
Undesirable tension that have built up can be released after each or several massive forming step(s) if the Al-based alloy(s) or multi-layered material is/are tempered after every overall forming process by at least 25% and at most 91% in one or more forming steps at a temperature in the range of between 85xc2x0 C. and 445xc2x0 C.